Such operating methods are known from DE-A-102 44 583 for example.
With the known operating methods for a sensor the sensor uses a detection facility to detect at least one analog signal, uses the at least one analog signal to determine at least one digital signal by means of an evaluation facility and transmits the at least one digital signal to a control facility outside the sensor. The at least one analog signal is also supplied to a monitoring facility of the sensor. The monitoring facility uses the analog signals supplied to it to monitor the correct operation of the detection facility. The monitoring facility only transmits an enable signal to the control facility outside the sensor, if the detection facility is operating correctly.
The sensor repeats the steps listed above regardless of whether or not it is in test mode. The sensor assumes test mode when a test signal is transmitted to it. In test mode the sensor manipulates the analog signal supplied to the monitoring facility in such a manner that the monitoring facility identifies that the detection facility is not operating correctly, when the monitoring facility is operating correctly.
A corresponding operating method for the control facility is also known from DE-A-102 44 583. In the context of this operating method the control facility receives the digital signal cyclically from the sensor. It checks whether an enable signal is also transmitted to it in addition to the digital signal. The control facility carries out these steps regardless of whether it is in a normal mode or a test mode. It repeats these steps cyclically.
In normal mode the control facility processes the at least one digital signal further when the enable signal is transmitted to it in addition to the at least one digital signal. Otherwise the control facility evaluates the transmitted digital signal as defective and outputs a message relating to a malfunction of the sensor.
From time to time the control facility transmits a test signal to the sensor and switches from normal mode to test mode. In test mode the control facility outputs a message relating to a malfunction of the sensor, when an enable signal is transmitted to it in addition to any digital signal transmitted during test mode. In test mode the control facility awaits the occurrence of an internal event inside the control facility and then switches automatically back from test mode to normal mode.
Electronic control facilities for industrial machines require a measured value supplied by a sensor at regular intervals, in other words cyclically, in order to be able to control a machine or system correctly. Examples of such control facilities are programmable logic controllers (PLC) and computer numerical controllers (CNC).
Earlier the analog signal detected by a detection facility of the sensor was itself first transmitted to the control facility for this purpose. With this type of data transmission it was possible—in some instances at least—for the control facility to monitor the correct operation of the sensor. This was particular true when the sensor transmitted a number of mutually correlated analog signals. One example of such mutually correlated analog signals is the sine and cosine signals of an incremental sensor, as these two signals have a 90° phase offset to one another and have essentially the same amplitude.
Sensors are also known, wherein digital signals derived from the analog signals are transmitted to the control facility in additional to the analog signals. With these sensors it is also possible—in some instances at least—for the control facility to monitor the sensor.
Sensors have become known more recently, which transmit the analog signals they detect to the control facility solely digitally. An example of such a sensor is described in the above-mentioned DE-A-102 44 583. With this sensor the control facility can no longer identify from the transmitted digital signal whether or not the sensor is operating correctly. Additional measures are therefore required in order to be able to identify any malfunction of the sensor. In particular a monitoring facility can be present in the sensor to monitor the detection facility. The monitoring facility and its operation are also described in the above-mentioned DE-A-102 44 583.